The field of art to which this invention is directed is foamed or cellular polymer compositions.
Foamed or cellular polymers have been commercially accepted in a wide variety of applications for a number of years. A foamed or cellular polymer or plastic is defined as a plastic the apparent density of which is decreased substantially by the presence of numerous cells disposed throughout its mass. The terms cellular plastic or polymer, foamed plastic or polymer, expanded plastic or polymer, and plastic or polymer foam have been used interchangeably to denote two-phase gas-solid systems in which the solid is continuous and composed of a synthetic polymer or rubber.
Polymers used in a large number of industrial foamed compositions are cellulose acetate, epoxy resins, styrene/polyester resins, phenolic resins, polyethylene, polystyrene, silicones, urea-formaldehyde resins, polyurethanes, latex foam rubbers, natural rubber, synthetic-elastomers, poly(vinyl chloride), ebonite, and polytetrafluoroethylene. Foamed polymers have been used for insulation, both sound and temperature, in furniture padding and in mattresses, as sponges, in packing materials, in plastic articles, as adhesives and the like.
Polyurethanes are used widely in foamed plastic applications even though isocyanates used in making polyurethanes present environmental problems. Polyurethanes find limited use in foamed adhesive compositions due to the fact that bonding of the polyurethane to a substrate requires, in most cases, a primer to enhance or maintain adhesion.
Epoxy-resin foams are characterized by good adhesive strength, which is important when foamed-in-place formulations are used, low water absorption, good dimensional stability, good heat resistance and, generally, good chemical resistance. The properties of rigid and semi-rigid epoxy foams are comparable to polyurethane formulations. However, because of the availability of lower cost foams with properties adequate for most commercial applications and because of the difficulties in achieving elastomeric epoxy systems, epoxy-resin foams are employed in somewhat specialized applications.
Acrylic resins have been used in hot melt thermoplastic adhesive systems. Hot melt thermoplastic adhesives are defined as solvent free adhesives which are applied in a molten state and which form a bond upon cooling to a solid state. A method for making foamed hot melt adhesives is described in U.S. Pat. No. 4,156,754. This method involves melting the thermoplastic resin, e.g., polymethyl methacrylate or polyethyl acrylate, dissolving a gas in the molten polymer under pressure, dispensing the melt under lower pressure so that the gas is released to form a foam, and compressing the hot foam between two substrates to force the gas from the foam and to form a bond with the substrates.
In U.S. Pat. No. 4,530,806, in the Background of the Invention, two processes for making foamed plastics are discussed. In one process, the starting composition which contains monomers and foaming agent is polymerized and foamed in a single stage process.
The other process is a two stage process again involving a composition containing monomers and foaming agent. In the first stage of this process, the monomers are polymerized without activating the foaming agent. In the second stage, the polymer is heated to a plastic or fluid condition, the foaming agent is activated, and the foamed plastic is formed.
U.S. Pat. No. 4,530,806 is directed to a process for making articles of foamed methyl methacrylate which involves (a) mixing methyl methacrylate, a plasticizing monomer, a polymerization catalyst, and a foaming agent which is not activated at the polymerization temperature; (b) polymerizing the monomers; and (c) heating the polymer to soften it and to activate the foaming agent, thereby forming a foamed article.
In U.S. Pat. No. 3,813,462, rigid acrylic polymer foams are prepared by polymerizing alpha-chloro-acrylic acid at room temperature, and then heating the polymer to generate an internal foaming agent through decomposition.
U.S. Pat. No. 3,804,684 discloses the formation of foamed plastics involving the extrusion of a polymer, e.g., polymethyl methacrylate, and a blowing agent.
In U.S. Pat. No. 3,673,126, a monomer, e.g., methyl methacrylate, is polymerized, the newly formed polymer is mixed with a foaming agent, and the mixture is extruded and foamed.
U.S. Pat. No. 3,470,277 describes a process for making cellular plastic products by soaking sheets of a polymer, e.g., polymethyl methacrylate, in a volatile liquid, stretching the sheets and heating them to form the foamed product.
A compressible cellular plastic material made from a polyacrylate elastomer is disclosed in U.S. Pat. No. 4,004,621.
Foamable, free-radical curable 2-component reactive resin compositions for use in fastening anchoring means in boreholes are described in U.S. Pat. No. 4,944,819. The disclosed reactive resin system is a styrene solution of an unsaturated polyester resin and a free radical catalyst.
In Kunstoffe 1978, 68(3), 141-4, Kircher and Preper describe the preparation of foamed plastics from blends of polymethyl methacrylate and polyurethanes made by polymerizing the urethane and acrylic monomers as a mixture.
There is a need for "foamed-in-place" cellular plastics which form strong adhesive bonds and which can be recycled for subsequent use.